1. Field of Invention
The invention relates to an image sensor, an image reading device, and a method for setting resolutions.
2. Description of Related Art
An image sensor is used for known facsimile machines, copiers, and hand scanners provided with an image reading device. FIG. 12 shows a structure of a known image sensor. Photoelectric converting elements P1a-P1e, such as phototransistors, output current (image signals) upon the detection of light. A power input terminal P2 inputs power supply voltage VDD. Channel selection switches P3a-P3e are connected to electric charge output ports of the photoelectric converting elements P1a-P1e. A shift register group P4 includes shift registers P4a-P4f and is started by a start signal SI input to a start signal input terminal P5. The shift register group P4 performs on-off control for the channel selection switches P3a-P3e in synchronization with clock pulse signals (CLK) input to a clock pulse signal input terminal P6, to sequentially output the image signal from each photoelectric converting element P1a-P1e, to an image signal output terminal P11, through a common signal line P7.
During the operation of the shift register group P4 after the shift register group P4 is started by the start signal SI, more specifically, during the time until the start signal SI input to the shift register 4a, is output from the shift register P4f, a flip-flop P8 continuously outputs an ON-signal. A chip selection switch P9 is interposed in the signal line P7 and open upon the reception of the ON-signal. A switch 10 is connected between the signal line P7 and a ground terminal P12. The switch 10 repeatedly closes or opens, in accordance with changes in the levels of the clock pulse signal (CLK).
Operations of a known image sensor are described below. The start signal SI and clock pulse signal CLK are input through the start signal input terminal P5 and the clock pulse signal input terminal P6, respectively, to the shift register group P4. The cycle of the start signal SI is set in order to be twice as long as that of the clock pulse signal CLK. The start signal SI is input to the shift register P4a at the falling edge of the clock pulse signal CLK. Upon the reception of the start signal SI, the shift register P4a is started. The shift register P4a closes the channel selection switch P3a during a time of one cycle of the clock pulse signal CLK. Accordingly, the image signal that the photoelectric converting element P1a outputs, is output to the image signal output terminal P11, through the signal line P7. Thereafter, the channel selection switch P3a is open again and the start signal SI input to the shift register P4a is transferred to the shift register P4b. 
Thus, the start signal SI is transferred to the shift registers P4b, P4c, P4d, P4e, in this order, so that the image signals from the photoelectric converting elements P1b-P1e are sequentially output to the image signal output terminal P11. The start signal SI is output, as a start signal for a sensor IC disposed downstream of the terminal P13 in a signal transmitting direction, from the shift register P4f, through a terminal P13.
For example, Japanese Laid-Open Patent Publication No. 5-227362 discloses an image sensor capable of setting two resolutions for image reading by selectively and switchingly outputting the current to an image signal output terminal from photocells or photoelectric converting elements using an externally input control signal.